Various principles and methods are known in the field of electronic or electro-optical distance measuring. One approach is to emit pulsed electromagnetic radiation, for example, laser light, onto a target to be measured and subsequently to receive an echo from this target as the backscattering object, wherein the distance to the target to be measured can be determined on the basis of the runtime of the pulse. Such pulse runtime meters have prevailed in many fields as standard solutions over time.
Two different approaches are typically used for detection of the backscattered pulse.
In the so-called threshold value method, a light pulse is detected when the intensity of the radiation incident on a detector of the distance meter used exceeds a specific threshold value. This threshold value prevents noise and interfering signals from the background from being incorrectly detected as a useful signal, i.e., as the backscattered light of the emitted pulse.
However, it is problematic that with weak backscattered pulses, as are caused by a greater measurement distances, for example, detection is no longer possible if the pulse intensity falls below the detection threshold, i.e., below the threshold value. The essential disadvantage of this threshold value method is therefore that the amplitude of the measurement signal has to be significantly greater than the noise amplitude of optical and electrical noise sources in the signal path, to sufficiently minimize incorrect detections, so that for measurements at relatively large distances, the threshold value method is only capable of limited use.
The other approach is based on the sampling of the backscattered pulse. An emitted signal is detected in that the radiation acquired by a detector is sampled, a signal is identified within the sampled range, and finally the location thereof is determined chronologically. Due to the use of a plurality of sampling values and/or addition of the received signal synchronous with the emission rate, a useful signal can also be identified under unfavorable circumstances, so that greater distances or background scenarios which are noisy or subject to interference can also be managed.
Currently, the entire waveform of the analog signal of the radiation acquired by a detector is frequently sampled in this case. After identification of the coding of the associated transmitted signal (ASK, FSK, PSK, etc.) of a received signal, a pulse runtime is determined very accurately from a defined progress point of the sampled and digitized signal, for example, the inflection points, the curve maxima, or integrally by means of an optimum filter known from time interpolation.
The limited linear modulation range of the electronic receiver circuit is problematic. At close range, the signal can saturate the receiver, so that the coding of the transmitted signal is no longer correctly ascertained or the runtime is determined insufficiently accurately.
One prohibitive disadvantage of signal sampling is thus that in the state of a saturated receiving electronics system due to excessively strong received light intensities, i.e., in particular in the case of short distances to the target object, suitably analyzable items of information of the measurement signal are no longer available, because then an actual signal profile can no longer be established as a result of detector saturation.
WO 2008/009387 describes in this case for pulse runtime measurements (ToF, Time-of-Flight) that alternatively—i.e., depending on which signal dynamic range of the receiver is addressed by the returning signal—either the threshold value method (with strong returning signal) or the sampling method WFD (with weaker returning signal) can be used.
A measurement method by means of signal sampling is known from U.S. Pat. No. 6,115,112, in which an approximate chronological establishment of the arrival time of the pulse is performed by a previously carried out coarse measurement. The actual distance measurement is then performed in the scope of a fine measurement for a further light pulse, the restricted possible arrival time of which is efficiently sampled.
Therefore, the measurement is allocated into a coarse measurement and a fine measurement. The application of this approach necessarily requires a sequential sequence of measurements, because a time window, in which the sampling measurements follow, is first defined by the threshold value measurement. A chronologically separated sequence of coarse and fine measurements on different pulses therefore takes place.
Either the restriction of the signal detection by a detection threshold or, however, the necessity of establishing a time window (coarse distance) for the sampling therefore represent the essential disadvantages of the above-described known measurement principles according to the pulse runtime principle.